This invention relates generally to heat exchangers and more particularly to double wall helically flighted heat exchangers.
Prior art heat exchangers are described in U.S. Pat. Nos. 4,316,502 and 4,434,539. These heat exchangers have been in widespread use in heat pump water heaters, desuperheaters, and water source heat pumps since 1980 and have performed very satisfactorily. They are highly efficient, relatively easy to manufacture and offer the unique feature of being classified as a double wall heat exchanger. The double wall feature is required by many building codes to isolate the refrigerant circuit from the water circuit to prevent the high pressure refrigerant and oil from entering the potable water line in the event of a rupture in a single wall heat exchanger. A break in either the refrigerant line or the water line allows the refrigerant or water to escape to the atmosphere rather than enter the other coil.
These prior art heat exchangers are typically made by winding at least two metal tubes (usually copper) that are to serve as the refrigerant and water tubes of the heat exchanger around a separate steel mandrel with steel end plates to which the wound tubes are clamped to prevent unwinding. During the winding process, the tubes collapse and then are expanded by internally pressurizing them to open the passageways back to the desired cross-sectional size. The size and shape of the tube coil is determined by the size and shape of the steel mandrel. This has limited the size and shape of the resulting heat exchanger. Moreover, all of the tubes in the coil had to have the same coiled configuration so that good heat transfer contact be maintained between the tubes.